1. Field of the Invention
The present invention relates to an edge-light type surface lighting device (e.g., an LCD panel lighting device) which is used for LCDs for, e.g., notebook personal computers and LCD TV sets.
2. Description of the Related Art
In recent years color LCDs have been widely used in various applications, including notebook personal computers, cellular phones and LCD TV sets. As information technology moves forward, LCDs have been required to have performance which fills particular needs in response to the increase in quantity of information to be managed, diversity of needs, and multimedia compatibility. A common challenge for manufacturers is to increase the brightness and the resolution of LCDs in order to meet such requirements.
LCDs are generally constructed in two parts: an LCD panel and a surface lighting device (back light unit). The construction of the surface lighting device can be divided roughly into two types: direct and edge light types. In the direct type, a light source (e.g., CCFL tubes) is placed under an emission plane. The Edge-light type of surface lighting devices use a straight lamp (CCFL lamp) along an end face of the LCD, two lamps on opposite end faces of the LCD, or L-shaped lamps along the opposite corners of the LCD. The edge-light type has become more widespread than the direct type because the edge-light type is advantageous for reducing the size of the LCD.
Mobile notebook personal computers and TV sets that adopt a color LCD generally use a battery (e.g., a rechargeable NiMH pack or a lithium-ion battery pack). The color LCD, especially the surface lighting device thereof, consumes most of the power of the battery. Accordingly, reducing power consumption of the surface lighting device as low as possible has been a significant challenge to manufacturers to extend battery life and improve the practical value of LCDs.
However, if a reduction in power consumption of the surface lighting device sacrifices the brightness thereof, the contrast becomes low, which is not preferable. It is desired to reduce power consumption of the surface lighting device while increasing the brightness thereof by improving optical efficiency of the surface lighting device.
In response to such a desire, a new type of surface lighting device has been developed. In this new type of surface lighting device, a lens unit is formed directly on a light exit surface of a light guiding plate (which serves an element of the surface lighting device), while a prism sheet, on which an array of prisms each having a triangular cross section is formed, is positioned on the light guiding plate so that the prism array faces the light guiding plate. According to this construction, light which is condensed by the lens unit that is formed on the light guiding plate is emitted obliquely from the light guiding plate and is subsequently directed toward the front of the device to thereby achieve a high brightness.
Another type of surface lighting device having a light guiding plate on which linear projections and depressions are formed to extend in a direction substantially perpendicular to the light incident surface of the light guiding plate has been developed. According to this structure, the spread of light in a direction parallel to the light incident surface of the light guiding plate is condensed by the lens effect that is produced by the linear projections and depressions to thereby achieve a high brightness.
If there is any defect on the screen of the LCD, the defect becomes an eyesore and thus is perceived as a serious defect since the user usually constantly looking at the screen. Such a defect can be in the form or, e.g., bright and dark streaks which occur in the vicinity of an end face of the light guiding plate. A part of the screen of the LCD which faces a non-light emitting portion of the surface lighting device, e.g., in the vicinity of either end of a straight lamp where an electrode portion thereof exits, is seen slightly darker than the remaining part of the screen, especially if the light guiding plate has a light incident surface which is longer than the length of a light emitting portion of the straight lamp.
In order to overcome this problem, it has been proposed that the light incident surface of the light guiding plate be formed as a mat surface so that the incident light thereon is diffused thereby to reduce the occurrence of the bright and dark streaks.
However, if the brightness of the surface lighting device is increased to meet the recent demand for the further enhancement of the brightness of LCDs, the aforementioned defect becomes more apparent.
In a typical edge-light type surface lighting device, a light bundle emitted from a straight lamp is made incident on an end face of a light guiding plate so as to exit from a light exit surface defined on one side of the light guiding plate. An LCD panel is positioned on the light exit surface with one or more diffusion sheets and/or prism sheets being positioned between the LCD panel and the light exit surface. Each prism sheet is a conventional optical element which is provided thereon with an array of minute parallel prism projections to have refractive and reflective functions. The light bundles emitted from the light exit surface of the light guiding plate are directed to the LCD panel via the prism sheet. Although one or two thicknesses of prism sheets are generally used, there is a limit of gathering the diffused light with the prism sheets.
An object of the present invention is to provide a surface lighting device which illuminates an LCD panel entirely with less unevenness in luminance distribution. Another object of the present invention is to provide a surface lighting device with a high brightness which improves the quality of images indicated on an LCD panel.
To achieve the object mentioned above, according to an aspect of the present invention, a surface lighting device is provided, including a light guiding plate having a light incident surface formed at one end surface of the light guiding plate and a light exit surface formed on a front surface of the light guiding plate; an elongated light source which faces the light incident surface; a light source reflector which reflects light emitted from the elongated light source toward the light incident surface; a prism sheet, wherein an array of minute parallel prism projections is formed on a surface of the prism sheet, the prism sheet being positioned so that the array of minute parallel prism projections faces the light exit surface; a regular reflection type reflector positioned to face a rear surface of the light guiding plate; a light guiding device for guiding light emitted from the elongated light source to each of two portions of the light incident surface which respectively face opposite ends of the elongated light source; and a diffuser, formed on the light incident surface, which diffuses incident light thereon.
In an embodiment, a diffusion reflector is formed on the regular reflection type reflector, which diffuses part of incident light on the regular reflection type reflector.
In an embodiment, the light incident surface is inclined with respect to a plane normal to the light exit surface.
Preferably, the light guiding plate includes a first array of minute parallel prism projections formed on one of the front surface and the rear surface of the light guiding plate to extend in a first direction, and a second array of minute parallel prism projections formed on the other of the front surface and the rear surface of the light guiding plate to extend in a second direction perpendicular to the first direction.
Preferably, the above-mentioned first direction is parallel to an axial direction of the elongated light source.
In an embodiment, the light guiding plate has a substantially wedge shape cross section, wherein a thickness of the light guiding plate gradually decreases in a direction away from the light incident surface. A first angle between the light incident surface and the light exit surface of the light guiding plate and a second angle between the light incident surface and the rear surface of the light guiding plate are an acute angle and either a right angle or an obtuse angle, respectively.
Preferably, the light incident surface, which is formed on one end surface of the light guiding plate, and the other end surface of the light guiding plate, which is positioned on the opposite side of the light guiding plate from the light incident surface thereof, are parallel to each other.
In an embodiment, the light source reflector includes a second diffuser which diffuses light emitted from the elongated light source.
In an embodiment, the light source reflector includes an extraordinary ray shield device which prevents light emitted from the elongated light source from being incident on the light exit surface from the front side thereof, and further prevents other light emitted from the elongated light source from being incident on the rear surface of the light guiding plate from the rear side thereof.
In an embodiment, the extraordinary ray shield device includes front and rear tongue portions formed integral with the light source reflector to extend in a direction away from the elongated light source along the light exit surface and the rear surface of the light guiding plate, respectively; and front and rear low-reflectivity areas positioned between the front tongue portion and the light exit surface of the light guiding plate and between the rear tongue portion and the rear surface of the light guiding plate, respectively.
In an embodiment, the front and rear low-reflectivity areas are formed on the front and rear tongue portions, respectively.
In an embodiment, the light guiding device includes two light guiding members fixed at the opposite ends of the elongated light source to surround two electrode portions formed at the opposite ends of the elongated light source, respectively.
In an embodiment, each of the two light guiding members is formed as a cylindrical cap to be fitted on a corresponding one of the electrode portions.
In an embodiment, the light guiding device includes two inclined surface portions formed on the light incident surface at opposite ends thereof in the vicinity of two electrode portions which are formed at the opposite ends of the elongated light source, respectively. Each of the two inclined surface portions is inclined with respect to the elongated light source to gradually approach a corresponding one of the electrode portions in a direction away from a central surface portion of the light incident surface.
In an embodiment, the light guiding device includes two arc surface portions formed on the light incident surface at opposite ends thereof in the vicinity of two electrode portions which are formed at the opposite ends of the elongated light source, respectively. Each of the two arc surface portions is curved to gradually approach a corresponding one of the electrode portions in a direction away from a central surface portion of the light incident surface.
In an embodiment, the light guiding device includes at least one first surface on which part of the light emitted from the elongated light source is incident, and at least one second surface which reflects the light incident on the at least one first surface to proceed toward the inside of the light guiding plate.
Preferably, the at least one first surface is angled relative to the at least one second surface by an angle of approximately 40 to 55 degrees.
Preferably, the at least one first surface and the at least one second surface constitute a plurality of first surfaces and a plurality of second surfaces, respectively, which form two sawtooth-like portions on the light incident surface at opposite ends thereof.
In an embodiment, each of the at least one first surface and the at least one second surface is formed as a mat surface.
In an embodiment, the light incident surface is formed as a mat surface.
Preferably, a central average surface height of the mat surface is in a range of 0.4xcexc to 1.0 xcexcm.
In an embodiment, the prism sheet includes an extraordinary ray shield device which prevents light emitted from the elongated light source from being incident on the light exit surface from front thereof.
In an embodiment, the prism sheet is slightly shifted relative to the light guiding plate in a direction away from the light incident surface so that a predetermined space is formed between the light shield strip and the light exit surface, wherein the extraordinary ray shield device includes a light shield strip which is positioned above the predetermined space to extend from one end of the prism sheet which faces the elongated light source.
Preferably, a width of the space between the light incident surface and the one end of the prism sheet, in a direction perpendicular to a longitudinal direction of the light incident surface and parallel to the light exit surface, is in a range of 0.5 mm to 10.0 mm.
In an embodiment, each of the light guiding plate and the regular reflection type reflector is formed as a rectangular plate; and the diffusion reflector includes two diffusion reflector surfaces formed on a front surface of the regular reflection type reflector at two corners thereof facing the opposite ends of the elongated light source, respectively.
In an embodiment, each of the two diffusion reflector surfaces includes an ink print pattern printed on the front surface of the regular reflection type reflector so that the degree of light diffusion of the diffusion reflector surface decreases in a direction toward a center of the front surface of the regular reflection type reflector, wherein ink having a high diffusion reflectance is used to print the ink print pattern.
In an embodiment, the diffusion reflector includes a diffusion reflector surface formed on a front surface of the regular reflection type reflector to extend parallel to the elongated light source along a side of the regular reflection type reflector which is adjacent to the elongated light source.
In an embodiment, the diffusion reflector surface includes an ink print pattern printed on the front surface of the regular reflection type reflector so that the degree of light diffusion of the diffusion reflector surface increases in a direction toward an end of the regular reflection type reflector which is adjacent to the elongated light source, wherein ink having a high diffusion reflectance is used to print the ink print pattern.
According to another aspect of the present invention, a surface lighting device is provided, including a light guiding plate having a light incident surface formed at one end surface of the light guiding plate and a light exit surface formed on a front surface of the light guiding plate; and an elongated light source which faces the light incident surface. The light incident surface is inclined with respect to a plane normal to the light exit surface.
Preferably, the light guiding plate includes a first array of minute parallel prism projections formed on one of the front surface and the rear surface of the light guiding plate to extend in a first direction, and a second array of minute parallel prism projections formed on the other of the front surface and the rear surface of the light guiding plate to extend in a second direction perpendicular to the first direction.
In an embodiment, the light guiding plate has a substantially wedge shape cross section, wherein a thickness of the light guiding plate gradually decreases in a direction away from the light incident surface. A first angle between the light incident surface and the light exit surface of the light guiding plate and a second angle between the light incident surface and the rear surface of the light guiding plate are an acute angle and either a right angle or an obtuse angle, respectively.
Preferably, the light incident surface, which is formed on the one end surface of the light guiding plate, and the other end surface of the light guiding plate, which is positioned on the opposite side of the light guiding plate from the light incident surface thereof, are parallel to each other.
The present disclosure relates to subject matter contained in Japanese Patent Applications Nos.2000-377388 (filed on Dec. 12, 2000) and 2001-225030 (filed on Jul. 25, 2001) which are expressly incorporated herein by reference in their entireties.